Conventionally, as a nonvolatile memory not loosing information stored therein even when a power supply is cut, a flash memory and a ferroelectric memory (FeRAM: Ferro-electric Random Access Memory) are known.
The flash memory includes a floating gate buried in a gate insulating film of an Insulated gate field effect transistor (IGFET) to store information by accumulating charges indicating information to be stored in the floating gate. For writing and deleting information, it is necessary to flow tunneling current passing through the insulating film, requiring a relatively high voltage.
FeRAM stores information using hysteresis characteristics (ferroelectric characteristics) of the ferroelectric. The ferroelectric capacitor structure having a ferroelectric film as a dielectric between a pair of electrodes generates polarization in accordance with a voltage applied between the electrodes, having spontaneous polarization even after the applied voltage is removed. When the polarity of the applied voltage is inversed, the polarity of the spontaneous polarization is inversed as well. Detection of this spontaneous polarization allows the information to be read out. As compared with the flash memory, the FeRAM operates at a lower voltage, having an advantage of allowing a high-speed writing while saving energy. A logic mixed chip (SOC: System On Chip) adopting the FeRAM for a conventional logic technology is under study for use in an IC card and the like.
At present, as a ferroelectric capacitor of a FeRAM, one adopting a so-called planer-type capacitor structure, in which a lower electrode of a capacitor and an impurity diffusion region of a transistor is eclectically connected by a drawn-out electrode provided on the lower electrode, is put into practical use. Recently, the semiconductor memories are increasingly demanded for microfabrication, and it is inevitable to reduce a memory cell area to respond to the demand. Accordingly, for a ferroelectric capacitor for a future FeRAM, an application of a so-called stack-type capacitor structure with its lower electrode being connected to the impurity diffusion region via a conductive plug to be a bulk contact immediately beneath the lower electrode is considered to be urgent.
Patent document 1: Japanese Patent Application Laid-Open No. 2002-33459
Patent document 2: Japanese Patent Application Laid-Open No. Hei10-50956.
In a stack-type ferroelectric capacitor, a lower electrode is formed in a stacked manner directly on the conductive plug (or, via a barrier conductive film formed as an oxidization preventing film of the conductive film) to connect the conductive plug and the lower electrode of the ferroelectric capacitor. In that case, the lower electrode (or the barrier conductive film, which is also applicable in the same manner hereinbelow) and those stacked thereafter inherit the impact of orientation and level difference of the conductive plug. The ferroelectric film formed on the lower electrode is strongly affected by the impact of the orientation and level difference and is thereby inevitably degraded by the impact of the conductive plug.
Particularly, tungsten (W) is frequently used as a material for the conductive plug used for the bulk contact. In a W plug, a groove-shaped recession called a seam is generated on the surface thereof to cause degasification backed by impurities accumulated in the seam, helping the ferroelectric film degrade further. Further, at the time of a surface planarization by a chemical mechanical polishing (CMP) method, a level difference generated between the surrounding insulating film is large, causing problems that the orientation of the ferroelectric film is significantly degraded and that the ferroelectric characteristics (a Q-switch characteristic and the like) are damaged.
Thus, the stack-type capacitor structure allows the reduction in the occupied area of the capacitor while assuring relatively large capacitance, and thereby the application of the structure to the ferroelectric capacitor contributes to elements in respect of the microfabrication and a density increase being demanded these days. Meanwhile, on the other side, due to the conductive plug provided as a bulk contact immediately beneath the lower electrode, the orientation of the ferroelectric film degrades significantly to damage the ferroelectric characteristics, being a serious problem left unsolved at present.